In many state of the art integrated circuits, emphasis is placed on providing signal amplification with low noise and low power dissipation. There are continuous demands for circuit improvements that enable higher functional throughput, increased on-chip signal processing, higher data rates, lower noise, less power dissipation, smaller cell size, and greater radiation hardness. In general, improved unit cell designs are needed to maximize functionality and provide high gain while minimizing noise, power dissipation, and cell size. Unfortunately, increased unit cell functionality usually requires greater circuit complexity, which leads to more noise and increased power dissipation. These conflicting requirements stretch the capabilities of state of the art electronic circuit designs.
Specifications for advanced Focal Plane Array (FPA) detectors, for example, require sensing of very low signal levels. For a detector system to maintain good sensitivity and resolution, the readout device of the FPA must amplify input signals received at the noise equivalent input (NEI) photon level so that the output signals are above the noise floor of the multiplexer and data processing electronics that follow the FPA. In some FPA applications, the sensor is required to perform in a low flux background while coveting a wide dynamic range. With these increasingly stringent specifications, state-of-the-art technology suffers from high stray capacitance and low source follower gain. The traditional approach to these problems has been simply to optimize the circuit layout and the device fabrication process.
A basic source follower circuit comprises an FET, with its drain biased at a fixed voltage, and a current load, which is generally supplied externally. Source followers are widely used as buffers to match a signal having a high output impedance and a load having a low input impedance. Although a source follower contributes significantly to the function of signal power amplification, it is generally not considered to be an amplifier because of its less than unity voltage gain characteristic.
Source follower operation using MOS technology is based on a single MOSFET, which is a four-terminal device. The gate-to-source voltage is the primary determinate of the conductivity between the drain and source. Because of the intimate physical contact between substrate and source, which actually forms a diode, the substrate potential with respect to the source affects the current because of a change in threshold voltage. This substrate-to-source voltage influence is known as the "body effect." In a basic source follower configuration with a constant current load (I.sub.L), a drain voltage (V.sub.D), a source voltage (V.sub.S), and an established FET gate bias (V.sub.g), as illustrated in FIG. 1A, gate and source voltages must track each other to maintain a constant current. As the gate voltage (V.sub.g) changes, the source voltage (V.sub.S) must change proportionally in the same direction (except for an offset caused by the body effect). If the gate is considered as an input and the source as an output of an electrical network, the source follows the gate's action. The body effect, however, causes a reduction in the V.sub.S changes with respect to the V.sub.g changes. Thus, the voltage gain is less than unity.
In some cases, when the substrate of the source follower FET is not shared with other FETs, it is possible to tie the source and substrate together, either directly, as illustrated in FIG. 1B, or indirectly through a voltage supply. In this configuration, the substrate-to-source voltage remains fixed and no reduction in voltage gain occurs from the body effect. However, the "source" (including the substrate) is physically much more massive and therefore requires more power to change its potential. This results in a trade-off of speed versus power. Furthermore, an isolated substrate structure is generally not supported by fabrication process technology. In another configuration, as illustrated in FIG. 1C, a resistive load (R.sub.L) can be used for convenience instead of a current load. A resistive load, however, contributes to gain degradation. Because the drain voltage is held constant, a secondary conductivity change, due to the FET channel length modulation when V.sub.S changes with respect to V.sub.D, also leads to gain degradation. All of these effects can be minimized using circuit design techniques, but minimization of the body effect is technologically limited. Therefore, the best source follower gain value achievable in the prior art is approximately 0.9.
In MOS technology, the source follower is a very popular electrical circuit building block because of its simplicity and it functional capability. However, the less than unity gain performance adds to noise susceptibility because signals are suppressed more than noise injected into the signal path. Furthermore, the inherent gain degradation limits the number of source follower stages that can be used in a circuit before signals have to be increased by a greater than unity voltage amplifier. An added amplifier, however, significantly increases cell size and complexity as well as power dissipation. The amplifier also tends to behave as an additional noise source. Thus, an amplifier, which is added to improve noise performance, is somewhat self-defeating in that it contributes to the noise of the circuit. In view of these factors, there is an identified need for a source follower circuit that has gain performance greater than unity and that does not add to overall circuit noise and power requirements.